Fix sigaction Undefind Behavior

Rust does not allow a `fn`-pointer to be null. This fixes that, while
luckily doing it in a way that leaves system calls
backwards-compatible :)

src/context/signal.rs

@@ -15,7 +15,7 @@ pub extern "C" fn signal_handler(sig: usize) {
         actions[sig]
     };
 
-    let handler = action.sa_handler as usize;
+    let handler = action.sa_handler.map(|ptr| ptr as usize).unwrap_or(0);
     if handler == SIG_DFL {
         match sig {
             SIGCHLD => {